Projection screens, particularly rear, transmission, or back projection screens in which modulated light from a projector is illuminated on the side facing away from the viewer and that light is viewed on the side facing the viewer, are used mainly for presentations, for showing slides, films, and video, for television sets and the like.
U.S. Pat. No. 6,839,167, Eckelt, et al., describes a projection screen that allows viewing of the projected image on the side of the screen facing away from the projector as well as on the side facing the projector. The screen is said to have only a low flicker effect and no light spots or places with excessive gloss. The screen has a first glass plate facing the projector that has a translucent ceramic coating, a second glass plate facing away from the projector that has a surface roughness to prevent reflections, and a white translucent polyvinyl butyral (PVB) sheet therebetween that is said to provide dispersion and partial reflection of the light to achieve best possible reproduction of the image. However, designing the screen to both reflect and transmit light so as to allow viewing from both sides degrades the brightness and contrast of the image to the viewer. It is also not practical to view projected information and data such as text when it is displayed backwards, as would be the case depending on which side of the screen you are viewing.
In the case of a rear projection screen, it is desirable to have high light transmission by scattering and diffusing forward as much light as possible while limiting the amount of light reflection backwards. It is also desirable that the light be spread over a wide viewing area with no visible concentration or hotspot when viewing the screen. It is also desirable to have the projection screen limit ambient light reflecting off the screen so as not to degrade the image contrast. The projection screen must also be able to resolve the individual picture elements or pixels projected. Thicker sheets of polymer (e.g., greater than about 0.060 inches [about 1.524 mm] thick) such as those available for light diffusion with diffusion particles dispersed throughout the sheet appear milky and will not exhibit sufficient resolution or contrast. It is desirable that the screen be easy and inexpensive to manufacture as well as scalable to very large sizes. The screen should also be durable, easy to clean, and able to withstand outdoor exposure.
Advances in projector resolutions are starting to render screen designs, such as plastic lenticular sheets disclosed in U.S. Pat. No. 4,919,515, Hasegawa, et al., more difficult and expensive to manufacture. The lens structure and pitch must be made smaller to accommodate higher resolutions; and they are also prone to moiré, speckle and other screen artifacts. Rear projection screens of the glass beaded type, such as disclosed in U.S. Pat. No. 2,378,252, Staehle, et al., while having good brightness and contrast suffer from an excess amount of mottle or blotch due to variations in the bead diameters. This causes the bead to have incomplete depth in the light absorbing layer and uneven light transmission across the screen. Surface relief holograms, such as disclosed in U.S. Pat. No. 5,609,939, Petersen, et al., while being high in light transmission and able to spread projected light over a wide area, are difficult and expensive to manufacture in larger sizes and are prone to pin hole defects causing excessive bright spots where the light is not diffused.
Many current PVB sheets designed to diffuse light do so by incorporating fine particles or pigments of a different refractive index, such as calcium carbonate, barium sulfate, silica, or quartz, into the PVB resin matrix. Particle size, morphology of the particle, concentration of particle to carrier resin, and the ratio of the refractive indices of the particle and the carrier resin all determine the quantity of scattered light. Typically, the particle size is between about 0.1 and about 10 microns. Too small of a particle size will become transparent to light and will not diffuse. Too large of a particle size and diffusion efficiency diminishes. Spherical particles are preferred as they exhibit a constant size to the projected light no matter what their orientation is within the carrier resin. This results in a more uniform and smooth appearance to the screen. If the contrast of the refractive indices is excessive, more light is reflected back and less is scattered forward. If the contrast is too low, the light is transmitted without being sufficiently scattered or diffused. Typically, the difference in refractive indices between the particles and the carrier resin is from about 0.06 to about 0.14. Particles of a lower refractive index than the resin are preferred. Particles having a higher index than the carrier resin will exhibit more light backscatter. The pigments should also be transparent or translucent so as not to be too opaque to light. Optically dense pigments such as titanium dioxide have a significant amount of light reflection and do not sufficiently scatter and diffuse the light forward. Some pigments are also difficult to disperse into the polymer resin matrix, or have a high specific gravity and settle out of dispersion. In other pigments, the morphology of the particle can significantly raise the viscosity of the resin matrix and make processing difficult. Selection of both pigment and carrier resin that maintains polarity of projected light is also desirable. There are applications with projection displays that utilize polarization of light to separate modulated light signals to two channels or views, one for the left and one for the right eye to show 3D images to the viewer. A polarization extinction ratio of at least about 25:1 is desirable to minimize ghosting or leaking from one channel to the other channel. Extinction ratios of 100:1 and above are preferred. Polymers such as polycarbonate or polyethylene terephthalate (PET) will depolarize light as will pigment such as calcite crystals.
Thus, there is a continuing need for improved polymer sheets that provide good light diffusion with an even appearance and minimal screen artifacts when used in projection screens.